Stir together sugar, milk, cocoa, corn syrup, and salt in a 2-quart saucepan. Bring mixture to a boil over medium-high heat, and cook until a candy thermometer registers 240°. Remove mixture from heat; add butter, and let melt. (Do not stir.) Let cool 10 to 15 minutes or until pan is cool to the touch. Stir in vanilla.
Beat mixture with an electric mixer at medium-low speed 2 to 3 minutes or until mixture begins to lose its gloss. Working quickly, pour fudge onto a buttered 11- x 7-inch platter. Let cool 15 minutes. Cut into 1-inch pieces.